This invention relates to the transmission of digital data in multiplexed form.
It has been proposed for use in conjunction with a television transmission system, to transmit several digital signals carrying sound information by means of a multiplexed data signal which involves forming blocks of consecutive sound samples, together with additional data for error protection purposes, into "packets" of data. Each packet is preceded by a "header" which contains the address of the packet which indicates the service to which it belongs.
The proposal is for the packet to begin with a "packet-type" (PT) byte, which has three readily-distinguishable assigned states known as BC1, BC2, and BI. The packet type byte BI precedes data which (exceptionally) does not represent sound samples, but which provides information about the technical configuration and program content of the sound channel carrying that address. This also allows information about the next use of that channel to be sent in advance for storage at a decoder.
The instant at which the change to new information is made, is signalled by changing the packet type byte preceding the sound data from BC1 to BC2, or vice versa, and it retains this changed state until the next change in applicable information.
The positioning of the successive packets of a channel within the multiplex is not regular but is essentially random. The packets are inserted in such a manner that a decoder which has a specified amount of buffer storage (for example 30 ms per sound channel) can smooth out the irregularities in the delivery of the sound samples adequately to provide a regular output sample rate to the subsequent signal processing circuit such as a digital to analogue converter. Furthermore there is no control between the time of delivery of the sound samples, and that of the associated vision signal. This is particularly important as a mis-timing of more than about 10 ms can result in a noticeable loss of "lip synchronisation".
There is therefore a need to find a way by which the different sound channels within a packet multiplex accompanying a television signal may be precisely co-timed on a sample-by-sample basis with one another, and to allow all of these channels to be referred to the timing of the associated television signal.